Nearly 30 million Americans suffer from a hearing deficit, at an annual cost to society of over $50 billion, it is incumbent upon us to understand the underlying causes of hearing impairment in order to reduce these significant personal and financial burdens. Whereas age-related hearing loss is the prevailing form of impairment, many other factors, including environmental and pharmacological insults and inherited genetic defects, cause hearing and balance dysfunction. Damage to the sensory cells of the inner ear is a frequent underlying cause. Hair cells, the sensory receptors of the inner ear, detect sound, linear acceleration, and angular velocity. The hair cell has a unique mechanoreceptive organelle, the hair bundle, which sits atop the cell?s apical surface. When a hair bundle is positively deflected, transduction channels open triggering mechanoelectrical transduction. A properly functioning hair-bundle is crucial for the overall function of the hair cell. Mutations in some hair-bundle proteins have already been identified as causative factors of hearing and vestibular defects. One such protein is the plasma-membrane calcium-ATPase (PMCA). This Ca2+/H+ exchanger plays a prominent role in Ca2+ regulation in the hair bundle. However, for every Ca2+ removed, one ATP is hydrolyzed and there is a reciprocal increase in intracellular H+ concentration. Therefore, ATP synthesis and H+ regulation are crucial for PMCA activity. Using pharmacology, molecular biology, biochemistry, and computational modeling, the goal of this proposal is to identify and characterize the mechanism by which the hair-cell bundle regulates H+ as a consequence of PMCA activity and to determine the source of ATP which fuels not only PMCA but other bundle ATPases.